arXiv subjects

K. Johnson

Publications and source records attributed to K. Johnson.

9 recordsLinked to original sources

arXiv2025

Enhancing Particle Identification in Helium-Based Drift Chambers Using Cluster Counting Insights from Beam Test Studies

Particle identification in gaseous detectors traditionally relies on energy loss measurements (dE/dx); however, uncertainties in total energy deposition limit its resolution. The cluster counting technique (dN/dx) offers an alternative approach by exploiting the Poisson-distributed nature of primary ionization, providing a statistically robust method for mass determination. Simulation studies with Garfield++ and Geant4 indicate that dN/dx can achieve twice the resolution of dE/dx in helium-based drift chambers. However, experimental implementation is challenging due to signal overlap in the time domain, complicating the identification of electron peaks and ionization clusters. This paper presents novel algorithms and modern computational techniques to address these challenges, facilitating accurate cluster recognition in experimental data. The effectiveness of these algorithms is validated through four beam tests conducted at CERN, utilizing various helium gas mixtures, gas gains, and wire orientations relative to ionizing tracks. The experiments employ a muon beam (1 GeV/c to 180 GeV/c) with drift tubes of different sizes and sense wire diameters. The analysis explores the Poisson nature of cluster formation, evaluates the performance of different clustering algorithms, and examines the dependence of counting efficiency on the beam particle impact parameter. Furthermore, a comparative study of the resolution achieved using dN/dx and dE/dx is presented.

physics.ins-det↗

arXiv2017

The role of molecular gas in galaxy transition in compact groups

Compact groups (CGs) provide an environment in which interactions between galaxies and with the intra-group medium enable and accelerate galaxy transitions from actively star forming to quiescent. Galaxies in transition from active to quiescent can be selected, by their infrared (IR) colors, as canyon or infrared transition zone (IRTZ) galaxies. We used a sample of CG galaxies with IR data from the Wide Field Infrared Survey Explorer (WISE) allowing us to calculate the stellar mass and star formation rate (SFR) for each galaxy. Furthermore, we present new CO(1-0) data for 27 galaxies and collect data from the literature to calculate the molecular gas mass for a total sample of 130 galaxies. This data set allows us to study the difference in the molecular gas fraction (Mmol/Mstar) and star formation efficiency (SFE=SFR/Mmol) between active, quiescent, and transitioning (i.e., canyon and IRTZ) galaxies. We find that transitioning galaxies have a mean molecular gas fraction and a mean SFE that are significantly lower than those of actively star-forming galaxies. The molecular gas fraction is higher than that of quiescent galaxies, whereas the SFE is similar. These results indicate that the transition from actively star-forming to quiescent in CG galaxies goes along with a loss of molecular gas, possibly due to tidal forces exerted from the neighboring galaxies or a decrease in the gas density. In addition, the remaining molecular gas loses its ability to form stars efficiently, possibly owing to turbulence perturbing the gas, as seen in other, well-studied examples such as Stephan's Quintet and HCG~57. Thus, the amount and properties of molecular gas play a crucial role in the environmentally driven transition of galaxies from actively star forming to quiescent.

astro-ph.GA↗

arXiv2015

Constraining globular cluster formation through studies of young massive clusters - V. ALMA observations of clusters in the Antennae

Some formation scenarios that have been put forward to explain multiple populations within Globular Clusters (GCs) require that the young massive cluster have large reservoirs of cold gas within them, which is necessary to form future generations of stars. In this paper we use deep observations taken with Atacama Large Millimeter/sub-millimeter Array (ALMA) to assess the amount of molecular gas within 3 young (50-200 Myr) massive (~10^6 Msun) clusters in the Antennae galaxies. No significant CO(3--2) emission was found associated with any of the three clusters. We place upper limits for the molecular gas within these clusters of ~1x10^5 Msun (or <9 % of the current stellar mass). We briefly review different scenarios that propose multiple episodes of star formation and discuss some of their assumptions and implications. Our results are in tension with the predictions of GC formation scenarios that expect large reservoirs of cool gas within young massive clusters at these ages.

astro-ph.GA↗

arXiv2014

TiNy Titans: The Role of Dwarf-Dwarf Interactions in the Evolution of Low Mass Galaxies

We introduce TiNy Titans (TNT), the first systematic study of star formation and the subsequent processing of the interstellar medium in interacting dwarf galaxies. Here we present the first results from a multiwavelength observational program based on a sample of 104 dwarf galaxy pairs selected from a range of environments within the SDSS and caught in various stages of interaction. The TNT dwarf pairs span mass ratios of M1/M2 <10, projected separations <50 kpc, and pair member masses of 7< log(M_*/Msun) <9.7. The dwarf-dwarf merger sequence, as defined by TNT, demonstrates conclusively and for the first time that the star formation enhancement observed for massive galaxy pairs also extends to dwarfs. Star formation is enhanced in paired dwarfs in otherwise isolated environments by a factor of 2.3 (+/- 0.7) at pair separations < 50 kpc relative to unpaired analogs. Starbursts, with Ha EQW >100 A, occur in 20% of TNT dwarf pairs, regardless of environment, compared to only 6-8% of matched unpaired dwarfs. Starbursts can be triggered throughout the merger (out to large pair separations) and not just approaching coalescence. Despite their enhanced star formation, most TNT dwarf pairs have similar gas fractions relative to unpaired dwarfs. Thus, there may be significant reservoirs of diffuse, non-starforming gas surrounding the dwarf pairs or the gas consumption timescales may be long in the starburst phase. The only TNT dwarf pairs with low gas fractions (fgas <0.4) and the only dwarfs with Ha EQW <2 A are found near massive galaxy hosts. Thus, dwarf-dwarf interactions are significant drivers of dwarf galaxy evolution, but ultimately environment is responsible for the quenching of star formation. This work is a precursor to an ongoing high resolution HI and optical imaging program to constrain the spatial distribution of star formation and gas throughout the dwarf-dwarf merger sequence.

astro-ph.GA↗

arXiv2013

Quantum Control of Qubits and Atomic Motion Using Ultrafast Laser Pulses

Pulsed lasers offer significant advantages over CW lasers in the coherent control of qubits. Here we review the theoretical and experimental aspects of controlling the internal and external states of individual trapped atoms with pulse trains. Two distinct regimes of laser intensity are identified. When the pulses are sufficiently weak that the Rabi frequency $\Omega$ is much smaller than the trap frequency $\otrap$, sideband transitions can be addressed and atom-atom entanglement can be accomplished in much the same way as with CW lasers. By contrast, if the pulses are very strong ($\Omega \gg \otrap$), impulsive spin-dependent kicks can be combined to create entangling gates which are much faster than a trap period. These fast entangling gates should work outside of the Lamb-Dicke regime and be insensitive to thermal atomic motion.

physics.atom-ph↗

arXiv2007

The Solar Wind Around Pluto (SWAP) Instrument Aboard New Horizons

The Solar Wind Around Pluto (SWAP) instrument on New Horizons will measure the interaction between the solar wind and ions created by atmospheric loss from Pluto. These measurements provide a characterization of the total loss rate and allow us to examine the complex plasma interactions at Pluto for the first time. Constrained to fit within minimal resources, SWAP is optimized to make plasma-ion measurements at all rotation angles as the New Horizons spacecraft scans to image Pluto and Charon during the flyby. In order to meet these unique requirements, we combined a cylindrically symmetric retarding potential analyzer (RPA) with small deflectors, a top-hat analyzer, and a redundant/coincidence detection scheme. This configuration allows for highly sensitive measurements and a controllable energy passband at all scan angles of the spacecraft.

astro-ph↗

arXiv2005

Microscopic and macroscopic compaction of cohesive powders

A novel method to investigate the compaction behaviour of cohesive powders is presented. As a sample, a highly porous agglomerate formed by random ballistic deposition (RBD) of micron sized spherical particles is used. A nanomanipulator deforms this small structure under scanning electron microscope observation, allowing for the tracking of individual particle motion. Defined forces are applied and the resulting deformations are measured. The hereby obtained results are compared to results from threedimensional discrete element simulations as well as macroscopic compaction experiments. Relevant simulation parameters are determined by colloidal probe measurements.

cond-mat.soft↗

arXiv1993

The Hidden Spatial Geometry of Non-Abelian Gauge Theories

The Gauss law constraint in the Hamiltonian form of the $SU(2)$ gauge theory of gluons is satisfied by any functional of the gauge invariant tensor variable $ϕ^{ij} = B^{ia} B^{ja}$. Arguments are given that the tensor $G_{ij} = (ϕ^{-1})_{ij}\,\det B$ is a more appropriate variable. When the Hamiltonian is expressed in terms of $ϕ$ or $G$, the quantity $Γ^i_{jk}$ appears. The gauge field Bianchi and Ricci identities yield a set of partial differential equations for $Γ$ in terms of $G$. One can show that $Γ$ is a metric-compatible connection for $G$ with torsion, and that the curvature tensor of $Γ$ is that of an Einstein space. A curious 3-dimensional spatial geometry thus underlies the gauge-invariant configuration space of the theory, although the Hamiltonian is not invariant under spatial coordinate transformations. Spatial derivative terms in the energy density are singular when $\det G=\det B=0$. These singularities are the analogue of the centrifugal barrier of quantum mechanics, and physical wave-functionals are forced to vanish in a certain manner near $\det B=0$. It is argued that such barriers are an inevitable result of the projection on the gauge-invariant subspace of the Hilbert space, and that the barriers are a conspicuous way in which non-abelian gauge theories differ from scalar field theories.

hep-th↗

arXiv1992

A Cutoff Procedure and Counterterms for Differential Renormalization

Explicit divergences and counterterms do not appear in the differential renormalization method, but they are concealed in the neglected surface terms in the formal partial integration procedure used. A systematic real space cutoff procedure for massless $ϕ^4$ theory is therefore studied in order to test the method and its compatibility with unitarity. Through 3-loop order, it is found that cutoff bare amplitudes are equal to the renormalized amplitudes previously obtained using the formal procedure plus singular terms which can be consistently cancelled by adding conventional counterterms to the Lagrangian. Renormalization group functions $β(g)$ and $γ(g)$ obtained in the cutoff theory also agree with previous results.

hep-th↗